The present invention generally relates to a process for producing a powder of perovskite-type ferroelectric materials and more particularly to a solution precipitation process for producing perovskite-type ferroelectric precursor powders, the calcined ferroelectric powder, and subsequent sintered ferroelectric materials.
Both dry and wet processes are conventionally known for producing perovskite-type ABO.sub.3 oxide powders, such as lead zirconate titanate, PZT, Pb(Zr.sub.1-x Ti.sub.x)O.sub.3,0,&lt;x&lt;1; where A.dbd.Pb and B.dbd.Zr,Ti!. The most common method for preparing these ferroelectric materials is a dry process by mixing the individual oxide forms of the constituents (e.g., for PZT: PbO.sub.1 TiO.sub.2, and ZrO.sub.2) in the desired ratio. The mixture is then heated in a process known as calcination to convert the constituent oxides into the desired perovskite phase. The perovskite phase is formed by the thermally induced diffusion and reaction of the component oxides to form the more thermodynamically stable complex oxide phase. For example, Terada et al, U.S. Pat. No. 5,180,699, issued on Jan. 19, 1993, describe a 2-stage calcination process for making PbA.sub.x Nb.sub.1-x O.sub.3, wherein a dried mixture of metals is mixed with a niobic acid sol and calcined at a temperature of 400-900.degree. C., adding a Pb compound and again calcining the resulting mixture.
In the dry process, it is difficult to obtain a powder material having a uniform composition and the powder material obtained may contain unwanted phases such as pyrochlore phases, zircona, titania, and lead oxide whose dielectric properties are inferior to the desired perovskite phase. To reduce the amount of unwanted phases and increase the desired perovskite phase which exhibits ferroelectric characteristics, higher calcination temperatures are used, causing the powder material to coarsen by formation of hard agglomerates due to partial sintering. Further crushing and mixing are then required to obtain fine-particle material necessary for many applications. The step of recrushing and remixing not only increases the manufacturing cost but also reduces the reliability of the final product by letting impurities get into the calcined product.
Because of the difficulty in controlling compositional homogeneity, powder particle size, and the uniform incorporation of low level dopants by the method, alternate methods of powder preparation have been developed. The alternate methods are wet processes that have been developed based on the use of solutions containing some or all of the A-site and B-site components of the oxide to be prepared. The solution approach allows for better mixing of the constituents, less dependence on the source of the constituents, and the possibility of producing a much finer powder with more uniform properties.
The wet process is often a coprecipitation method in which a mixed solution of all of the components which constitute the desired perovskite phase is prepared, a precipitate-forming liquid such as an alkali is added for effecting coprecipitation, and the precipitate is separated out, dried and calcined. The solutions utilized in the wet process can be aqueous or nonaqueous solutions as well as sol-gels and slurries. This type of processing can lead to the production of fine powders that do not require additional crushing and grinding.
Oda et al., U.S. Pat. No. 4,874,598, issued on Oct. 17, 1989, and Watanabe et al., U.S. Pat. No. 5,229,101, issued on Jul. 20, 1993, describe an aqueous solution method of producing a perovskite-type oxide of the ABO.sub.3.
Miyauchi et al, U.S. Pat. No. 4,019,915, issued on Apr. 26, 1977, describe a slurry method of preparing perovskite-type materials with the general formula ABO.sub.3 by taking a solid solution of A, dispersing the elements of component B into the solution, admixing oxalic acid in the presence of an alcohol to form a precipitate, adjusting the pH of the slurry to complete the precipitation process, drying the precipitate into a powder, calcining the powder and molding the powder under pressure.
Woodhead et al., U.S. Pat. No. 5,091,348, issued on Feb. 25, 1992, describe a sol-gel method of making a perovskite-type structure by mixing non-alkoxide sols with a metal salt, dehydrating the mixed sol to form a gel, and heating to form a perovskite-type product that can be comminuted.
Haertling and Land (Ferroelectrics, 1972, Vol. 3, pp. 269-280) describe a sol-gel method of making perovskite-type materials by mixing alkoxide solutions of zirconium and titanium with a lead oxide powder and lanthanum acetate solution, hydrolyzing the solution to form a white-colored solution slurry of paint-like consistency, drying the slurry, crushing the dried product, calcining the product and then crushing and calcining again to obtain a fine-grained powder product.
Ramamurthi and Payne (J. Am. Ceram. Soc., 1990, 73, 2547) describe a sol-gel method for making PT and PZT materials using alkoxides and acetates prepared in a methoxyethanol solvent system. The processing requires several hours as well as multiple processing steps to get the metal compounds in solution. These steps include refluxing at elevated temperature and hydrolysis.
Although a wet process synthesis approach for producing perovskite-type powders offers many potential advantages, there are often increased costs related to its use. The precursor chemicals used are often more expensive than the traditionally used metal oxides. Additional up-front processing steps are required to prepare solutions. Liquid wastes are generated that must be dealt with. Also, depending on the material being prepared and the precursor solutions used, large diameter particles are often obtained as the particles tend to coagulate during the formation of precipitate, drying or calcination. When this occurs, the method generally requires that the product be crushed and mixed again to produce the necessary fine, homogeneous particles before they are molded and sintered. The step of recrushing and remixing not only increases the manufacturing cost but also reduces the reliability of the final product by letting impurities get into the calcined product.
The need for perovskite-type ABO.sub.3 oxide powders with tightly controlled physical and chemical properties for use in preparing high quality electronic ceramics has led to the development of these various solution synthesis routes to powders with their various shortcomings and drawbacks.
It would be more desirable, for the routes based on precipitation, to have a process that is simple with respect to the number of solutions that need to be mixed to form the precipitate. In such a process, one solution could contain all of the A and B components of the oxide to be produced and a second solution could contain a precipitant. Mixing of the two solutions could quantitatively remove the A and B solution species without the addition of a third solution, for example, to adjust pH. The precipitate product would be readily filterable and require little or no washing before drying. The dried precipitate powder would be easily converted by calcination to the desired perovskite phase at moderate temperatures where subsequent comminution and calcination steps are not required. The resulting oxide powder would be phase pure and reactive to sintering for subsequent formation of ferroelectric ceramic materials. Such a process is realized in the present invention.